I have a question concerning connecting two DVB-T dongles on the same clock
source for interferometric (or passive radar) measurements, as described at
http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html
I have assembled the same system with one dongle used as oscillator on a 28.8 MHz resonator
and the second one as a slave to this clock. All works fine, solved the issue
when the oscillator would not start, now I have a reliable source of measurements.
Initial tests (these are R820T-based dongles) exhibits significant random phase drift
which I attributed to heating of the chips (they get above 50 degC when running continuously),
so after gluing two heat sink with heat-conducting epoxy, I more or less managed to
get a stable phase measurement when recording a same oscillator (200 MHz) with the
two dongles and displaying the phase as angle(conjugate(signal1)*signal2).
The question is as follows: at http://jmfriedt.sequanux.org/ph_tout.pdf I have shown one
graph, quite representative of all my experiments, displaying the evolution of the phase
difference between both dongles connected to the same 200 MHz oscillator. I *always*
start with a quite stable phase difference (red curve -- inset in a zoom of this particular
measurement) after plugging in my USB hub fitted with the two dongles and starting gnuradio-companion
for recording the dongle I/Q stream (notice the abscissa sampling rate of 10 Hz => the full
graph is about 1-hour long). After about an hour, I stop recording the red curve, and
all I do is switch off gnuradio-companion and start it back => green curve with a quickly falling
phase. Switch off again, disconnect-reconnect USB hub, restart an acquisition => blue curve.
Same procedure => magenta curve.
Can anyone hint at an explanation as to why I always start with a reasonably stable phase
difference (yet not constant -- is the phase fluctuation indeed due to heating of the fractional
PLL in each RF frontend, drifting below the feedback loop time constant ?), but more worrisome
why I always get this huge drift after launching a new acquisition ? The fact that I always
get the same slope hints at a sofware/hardware communication issue, but how it is possible
since both dongles are clocked by the same source and receive the same commands from the
software ?
Thanks, JM
--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France